I tried to share my iPhoto library with putting it into my sharing folder. I changed the permissions, so the other users can use it. Now they can use it, but if they add new photos, i cannot edit them. They were displayed, but if I try to edit the, there is only a black screen instead of the picture.
Did you have a look at what Apple proposes in the corresponding support document?
It covers using an external hard drive to house one iPhoto Library as well as moving those files to the /Users/Shared folder on the Mac.